Conventional golf balls, solid or wound, typically have at least one core layer and at least one cover layer. Two-piece balls having a solid construction are popular with golfers because they provide a very durable ball with high initial velocity resulting in longer flight distance. Due to the rigidity of the materials used, however, the balls have a “hard” feel when struck with a club and a relatively low spin rate that makes them difficult to control, particularly on shorter approach shots.
Wound balls, i.e., spherical solid rubber or liquid center with a tensioned elastomeric thread wound thereon, are preferred by some golfers for a softer feel and higher spin enabling better control in and around the green. Wound balls typically travel a shorter distance, however, when struck as compared to a two piece ball. Moreover, as a result of their more complex structure, wound balls generally require a longer time to manufacture and are more expensive to produce than a conventional two piece ball.
Solid cores, used in wound or solid golf balls, are generally formed of a polybutadiene composition. In addition to one-piece cores, solid cores can also contain a number of outer layers, such as in a dual core golf ball. Covers, for solid or wound balls, are generally formed of ionomer resins, balata, polyurea, or polyurethane, and can consist of a single layer or include one or more layers, e.g., a dual cover with an inner and outer cover layer, and optionally at least one intermediate layer disposed about the core.
The core of solid golf balls is the “engine” of the ball, providing the velocity required for good distance. However, a core that is too hard can result in a golf ball that provides poor feel. Manufacturers are constantly experimenting with various core compositions and constructions in an effort to optimize both feel and distance. Most conventional solid cores comprise polybutadiene rubber or some modified form thereof, which provides the primary source of resiliency for the golf ball.
Polybutadiene rubber core compositions still have room for improvement in resilience, which is determined by coefficient of restitution (“COR”). As would be appreciated by those skilled in the art of golf ball manufacturing, the COR along with angle of trajectory (i.e., launch angle) and club head speed determine the distance a golf ball will travel when hit by a golf club. One way to measure the COR is to propel a ball at a given speed against a hard massive surface and measure its incoming and outgoing velocity. The COR is the ratio of the outgoing velocity to the incoming velocity and is expressed as a decimal between zero and one. There is no United States Golf Association limit on the COR of a golf ball, but the initial velocity of the golf ball cannot exceed 250+/−5 feet/second. As a result, the industry goal for initial velocity is 255 feet/second, and the industry strives to maximize the COR without violating this limit.
In general, polybutadiene rubbers of high molecular weight (high Mooney viscosity) have better resilience than polybutadiene rubbers of low molecular weight (low Mooney viscosity). However, as the molecular weight increases, the milling and processing properties of the polybutadiene rubber deteriorate. Polybutadiene rubber catalyzed with lanthanide series elements, such as neodymium, tends to be linear and narrow in polydispersity (closer to 1.0). The linearity and narrow polydispersity allows high-molecular weight neodymium-catalyzed polybutadiene rubber (“Nd-BR”) to provide improved resilience, but causes problems in extrusion processes and storage difficulties such as cold flow. In comparison to Nd-BR, polybutadiene rubber catalyzed with cobalt and/or nickel (“Co-BR, Ni-BR”) is typically more branched and has wider polydispersity (distant from 1.0). However, while the branching characteristic facilitates processing, the wide polydispersity and branching generally gives low resilience.
Attempts to improve golf ball COR by using various blends of polybutadiene rubber in core compositions include, among others, U.S. Pat. Nos. 4,683,257; 4,931,376; 4,955,613; 4,984,803; 5,082,285; 6,139,447; 6,277,920; 6,315,684; and 6,774,187.
While advances have been made, there remains a need in the golf ball manufacturing art to provide compositions with improved properties and processability that are useful in golf ball components and, more specifically, golf ball core components. Advantageously, the compositions of the present invention provide enhanced processing characteristics without sacrificing resilience in the resulting golf balls.